My only slightly educated guess is that Delta's pontification regarding low cost units of capacity has more to do with the anticipated blood bath that will happen as US Air slowly dies with bankruptcy protection to defer paying creditors and government loans to fund ongoing operations. Right now US Air overlaps 34% of Delta's route structure. We can expect fire sale US Air fares, as well as new market entrants who cut prices to gain market share.
Every airline is trying to decide how to step into US Air's market. Delta is a natural choice for this service, but AA, LUV, and to some extent AAI are not going to let Delta have it without a fight. Heck with the code share agreement, perhaps even UAL will enter the fray, but UAL can not afford the losses and has an uncompetitive cost structure.
The larger CRJ 700 & 900, as well as the E170 & 190 would be perfect equipment to take over US Air's market and yes, Delta is cost competitive with the Connection carriers and ALPA may very well allow mainline to reach a cost structure on these airplanes that would allow Delta an advantage (pure speculation, based party on Duane Woerth's "50 seat natural dividing line statements to AW&ST)
My bet is on AAI to be a long term player. The stock is a good buy right now. Delta's labor difficulties, the expense of the RJ's and bureaucratic momentum give AAI the advantage in this shake out. Delta can not afford to run two carriers into bankruptcy right now and the DOT would not (I don't think) allow Delta to purchase AAI due to the anti competitive effect on consumers.